1. Field of the Invention
The present invention relates to a photocoupler device integrally including a light emitting element, and an output photodetector and a monitor photodetector for receiving light emitted by the light emitting element. The present invention also relates to a fabrication method thereof.
In another aspect, the present invention relates to a lead frame for the photocoupler device.
2. Description of the Related Art
In a photocoupler device, an optical signal is transmitted from a light emitting element on a primary side to a photodetector on a secondary side while the primary side and the secondary side are electrically isolated from each other. The light emitting element and the photodetector are mounted on a lead frame, an optical path therebetween is made of a light-transmissive resin, and the optical path is covered with a light-shielding resin.
In recent years, a photocoupler device including two photodetectors, one for signal transmission and the other for monitoring, has been proposed. Specifically, an extra photodetector is provided on the primary side in order to monitor an emission output level of the light emitting element, and to feed the monitoring result back to the light emitting element. This solves the problem of nonlinearity in temperature characteristics, etc., which is specific to light emitting elements, thereby stabilizing the emission output level.
FIG. 10 is a plan view showing one example of the conventional photocoupler device. FIG. 11 is a cross-sectional view of the photocoupler device shown in FIG. 10. As shown in FIGS. 10 and 11, a light emitting element 101 is mounted on a primary side lead frame 102a via an electrically conductive paste or the like, and connected to a lead frame 103 for connecting a line by an Au wire 104 or the like. An output photodetector 105 is mounted on a secondary side lead frame 102b, and is connected to a lead frame 106 for connecting a line by an Au wire 104 or the like. A photodetector 107 for monitoring (hereinafter, referred to as xe2x80x9cmonitor photodetector 107xe2x80x9d) is mounted on the primary side lead frame 102a in the same manner as the light emitting element 101, and connected to a lead frame 108 for connecting a line by the Au wire 104 or the like.
The light emitting element 101, the output photodetector 105, and the monitor photodetector 107 are placed on the same plane, and are covered with a light-transmissive resin layer 109 which is made of a transmissive potting resin such as a silicone resin. Additionally, the resultant structure is covered with a molded layer 110 made of a light-shielding resin such as an epoxy resin, in order to reflect an optical signal from the light emitting element 101 and/or block interfering stray light from outside.
FIG. 12 is a schematic circuit diagram of the photocoupler device including the light emitting element 101, the output photodetector 105, and the monitor photodetector 107, which are electrically isolated from one another. Between the light emitting element 101 and the photodetector 105, and between the light emitting element 101 and the photodetector 107, only optical signals are transmitted.
In such a structure, upon receiving an electric signal through the lead frame 103 for connecting a line, the light emitting element 101 photoelectrically converts the electric signal to an optical signal, and emits the optical signal. The optical signal travels through the light-transmissive resin layer 109 and is reflected by the interface between the light-transmissive resin layer 109 and the molded layer 110. The reflected optical signal reaches the output photodetector 105 and the monitor photodetector 107. The output photodetector 105 converts the optical signal to an electric signal, and outputs the electric signal. Likewise, the monitor photodetector 107 converts the optical signal to an electric signal, and outputs the electric signal. The electric signal from the monitor photodetector 107 is fed back in order to control an emitting operation of the light emitting element 101.
Next, prior art directed to a lead frame for a photocoupler device is described.
FIG. 17 is a circuit diagram showing a configuration example of a high-linearity analogue photocoupler device (hereinafter, simply referred to as a xe2x80x9clinear photocouplerxe2x80x9d). Although not shown, two devices are required for the substitution of pulse transes. Thus, a majority of linear photocoupler devices include two channels of devices in one package.
A typical linear photocoupler includes a light emitting element (LED) 202 and a monitor output element (photodiode) 203 on a primary side, and an output element (photodiode) 204 on a secondary side. In the case where a current flowing through the light emitting element 202 on the primary side is represented by IF, and photoelectric currents flowing through the monitor output element 203 and the output element 204 are represented by IPD1 and IPD2, respectively, the relationships between IF, IPD1 and IPD2 are as follows:
IPD1=IFxc3x97K1, IPD2=IFxc3x97K2.
If K3=K2/K1, IPD2=IPD1xc3x97K3. It is desirable that K3 is as close to 1 as possible. xe2x80x9cK3=1xe2x80x9d is most desirable for facilitating the design of the peripheral circuits. That is, it is required to adjust the photoelectric currents flowing through the monitor output element 203 and the output element 204 to the same or substantially identical value (i.e., it is required that the elements 203 and 204 receives light from the light emitting element 202 at the same level). Furthermore, electrical insulation between the primary and secondary sides are required, which is an essential characteristic of the photocoupler device.
As described above, a typical photocoupler includes the light emitting element 202, the monitor photodetector 203, which is used for stabilizing the emission of the light emitting element 202, on the primary side and the output photodetector 204 on the secondary side. In such a device, it is required that the same level of light from the light emitting element 202 is incident on each of the two photodetectors 203 and 204, and that the primary side and the secondary side are electrically isolated from each other.
Hereinafter, an exemplary structure of the conventional linear photocoupler and an exemplary fabrication method thereof will be described with reference to FIGS. 18A, and 18B, 19, and 20.
Referring to FIGS. 18A (plan view) and 18B (cross-sectional view), a light emitting element 202, a monitor output photodetector 203, and an output photodetector 204 are die-bonded (adhered) onto a flat lead frame 201. After the elements are connected to the outer leads by gold wires 205, the elements are covered with a transparent silicone resin 206 or the like, and then transfer-molded with an epoxy resin 207.
FIG. 19 (an example of the structure of the lead frames) and FIG. 20 (a cross sectional view of an example of a photocoupler) show another example. In this example, lead frames 201 and 201xe2x80x2 are used. A tip of the lead frame 201 is raised upward and provided with only the light emitting element 202 adhered and mounted thereon, while a tip of the lead frame 201xe2x80x2 is lowered and provided with a photodetector 203 for monitoring and a photodetector 204 for output adhered and mounted thereon. Each element is wire bonded to the outer leads, respectively, as shown in the drawings. The light emitting element 202 is precoated with a transparent silicone resin 208 for relieving the stress thereof, and then positioned over the photodetector 203 for monitoring and the photodetector for output 204 so as to face the photodetectors 203 and 204.
Thereafter, the first transfer molding process is performed with a light-transmissive epoxy resin 209 and, in addition, the second transfer molding process is performed with a light-shielding epoxy resin 210, resulting in the structure shown in FIG. 20.
For the photocoupler device, the ratio of the output level between the monitor photodetector and the output photodetector, and the stability thereof are essential characteristics. Thus, various ideas and considerable efforts has been directed to the formation of the light-transmissive resin layer, i.e., the optical path which affects the ratio of output level between the photodetectors. For example, by adjusting the position of the light emitting element and/or the photodetectors with respect to the lead frame, or by utilizing a tension of the Au wire or the like electrically connecting each of the light emitting element and/or the photodetectors to the lead frame, the shape of the light-transmissive resin layer can be stabilized. Alternatively, by employing a silicone resin having high viscosity as a light-transmissive resin, the shape of the light-transmissive resin layer can be stabilized.
However, in the conventional photocoupler device, a light emitting element, a photodetector for output, and a photodetector for monitoring are placed on the same plane. In such a design, these elements consume a large area, and thus a great amount of silicone resin is required in order to cover the entire elements. Accordingly, using the silicone resin for the performance improvement of the photocoupler device leads to an increase of cost because the silicone resin is expensive.
Furthermore, as described above, the shape of the light-transmissive resin layer is stabilized by using a surface tension of the silicone resin caused by the shape of the Au wire or the like connecting the elements to the lead frame. Thus, when an unstable assembly and fabrication process causes the shape of the Au wire or the like to vary so as to change the surface tension of the silicone resin, the light-transmissive resin layer are deformed, and accordingly, the ratio in the output level between the photodetector for monitoring and the photodetector for output varies.
Moreover, since the light-transmissive resin layer and the molded layer provided thereon have different coefficients of thermal expansion, the state of the interface therebetween is unstable against temperature variation, and the state of reflection on the interface is also unstable. Thus, temperature variation changes the transmission efficiency of optical signals between the light emitting element and each of the photodetectors. Consequently, reliability of the feed back control based on the output from the photodetector for monitoring is degraded.
In a conventional photocoupler device, since the optical signal travels by using reflection, a white resin with less fillers has been typically used for the outer molded layer. Accordingly, the photocoupler device is likely to be affected by interfering stray light from outside, and thus has less reliability.
In the prior art described above with reference to FIGS. 18A and 18B, which is directed to the photocoupler device and the lead frame for photocoupler device, all of the three elements (i.e., the light emitting element 202, the monitor output element 203, and the output element 204) are required to be entirely covered with the silicone resin 206. In such a technique, it is very difficult to adjust the amount of resin, and to stabilize the shape of applied resin. Furthermore, when the amount and/or the shape of the silicone resin 206 is not uniform, it is impossible to transmit the same quantity of light to the monitor output element 203 and the output element 204. As a result, a difference in the amount of photoelectric current between the monitor output element 203 and the output element 204 becomes large. Therefore, the difference between K1 and K2 described above becomes large.
Furthermore, in this structure, characteristics of the element are likely to vary due to the temperature fluctuation around the element (caused by reflow, soldering, etc.). Specifically, because of heat in the interface between the silicone resin 206 covering the elements 202, 203, and 204 and the epoxy resin 207, the silicone resin 206 and the epoxy resin 207 are peeled off from or adhered to each other at the interface therebetween. This affects the reflection of light from the light emitting element 202, and therefore causes light-transmission efficiency from the element 202 to the monitor output element 203 and the output element 204 to be varied. In addition, the withstand voltage between the primary and secondary sides is inferior to that of the dual-transfer type photocoupler shown in FIGS. 19 and 20.
On the other hand, the prior art described above referring to FIGS. 19 and 20 is free from such problems. However, since the two lead frames 201 and 201xe2x80x2 are combined into a laminate, as shown in FIG. 21, leads protruding from the primary and secondary side lead frames of the package are not present on the same plane.
Furthermore, as shown in FIG. 21, a tie bar portion 211 is superposed on another tie bar portion 211xe2x80x2. Such unnecessary portions (shaded portions) of the tie bar 211 are cut off by a metal mold after being covered with light-shielding epoxy resin 210. In order to cut off the shaded portions, more pressure is required as compared with the case where single tie bar (having xc2xd the thickness of the shaded portion) is cut off. As a result, a greater impact is given to the elements and, in the worst case, causes the deformation of the leads, or cracks in the package. In addition, portions 211A of the tie bar 211 are not cut away and therefore remain on the leads. These remainders may be left in the mold unless manually removed. This may cause troubles such as breakage of the mold. When the remainders are manually removed, productivity significantly decreases.
As described hereinabove, the conventional photocoupler device bears various complicated problems as to characteristics, structure, and productivity thereof. In order to address such problems, it has been required to devise a photocoupler having a novel structure.
According to one aspect of the present invention, a photocoupler device includes a light emitting element; a monitor photodetector and an output photodetector for receiving light from the light emitting element; a primary side lead frame for mounting the light emitting element and the monitor photodetector; and a secondary side lead frame for mounting the output photodetector, wherein the light emitting element and the output photodetector are placed so as to face each other.
In one embodiment of the present invention, the monitor photodetector on the primary side lead frame and the output photodetector on the secondary side lead frame are provided on a same plane, and are positioned so as to face the light emitting element on the primary side lead frame.
In another embodiment of the present invention, the monitor photodetector and the light emitting element on the primary side lead frame are provided on a same plane, and are positioned so as to face the output photodetector on the secondary side lead frame.
In still another embodiment of the present invention, the monitor photodetector and the light emitting element of the primary side lead frame are positioned at different levels, and face the output photodetector on the secondary side lead frame.
In still another embodiment of the present invention, the light emitting element, the monitor photodetector, and the output photodetector are covered with a light-transmissive resin through a first transfer molding process, and then, are further covered with a light-shielding resin through a second transfer molding process.
In still another embodiment of the present invention, the light emitting element and the monitor photodetector, or the light emitting element and the output photodetector are covered with a single transparent resin layer.
In still another embodiment of the present invention, at least one of the light emitting element, the monitor photodetector, and the output photodetector is precoated with a transparent resin layer.
According to another aspect of the present invention, a method for fabricating a photocoupler device including a light emitting element, and a monitor photodetector and an output photodetector for receiving light from the light emitting element includes steps of: mounting the light emitting element on a first lead frame; mounting the output photodetector and the monitor photodetector on a second lead frame; and combining the first lead frame and the second lead frame so that the light emitting element and the monitor photodetector are provided on a primary side, and the output photodetector is provided on a secondary side.
According to still another aspect of the present invention, a lead frame for a photocoupler device includes a primary side lead frame and a secondary side lead frame, wherein the primary side lead frame and the secondary side lead frame are combined together, and have a common reference plane, wherein the primary and secondary side lead frames each have a plurality of turned-up portions extending upward with respect to the reference plane, and a plurality of turned-down portions extending downward with respect to the reference plane, and one of the turned-up portions does not extend over any other turned-up portion, and one of the turned-down portions does not extend below any other turned-down portion.
In one embodiment of the present invention, element-pairs of a light emitting element and a monitor output element, and output elements are provided one after another on each of the primary and secondary side lead frames; and the primary and secondary side lead frames are assembled so that one of the element-pairs provided on the primary side lead frame faces a corresponding one of the output elements provided on the secondary side lead frame, and one of the element-pairs provided on the secondary side lead frame faces a corresponding one of the output elements provided on the primary side lead frame.
According to still another aspect of the present invention, a photocoupler device includes a light emitting element; a monitor output element; an output element; and a primary side lead frame and a secondary side lead frame having a common reference plane, wherein the primary and secondary side lead frames each have a plurality of element mounting portions for alternately mounting the light emitting element, the monitor output element, and the output element, wherein some of the plurality of element mounting portions extend upward and others extend downward with respect to the reference plane, and wherein one of the element mounting portions extending upward does not extend over any other element mounting portion extending upward, and one of the element mounting portions extending downward does not extend below any other element mounting portion extending downward, and the primary and secondary side lead frames are assembled so that one of pairs of the light emitting element and the monitor output element provided on the primary side lead frame faces corresponding one of the output elements provided on the secondary side lead frame, and one of pairs of the light emitting element and the monitor output element provided on the secondary side lead frame faces corresponding one of the output elements provided on the primary side lead frame.
In the present invention having the above-described structure, as typically shown in FIGS. 1 and 2, upon receiving an electric signal via the lead frame 17 for connecting a line, the light emitting element 12 photoelectrically converts the electric signal to an optical signal, and outputs the optical signal. The optical signal travels through the light-transmissive resin 22 and reaches the output photodetector 15 and the monitor photodetector 13. The output photodetector 15 converts the optical signal to an electric signal, and outputs the electric signal via the lead frame 19 for connecting a line. Likewise, the monitor photodetector 13 converts the optical signal to an electric signal, and outputs the electric signal via the lead frame 18 for connecting a line. The electric signal from the monitor photodetector 13 is fed back in order to control the output operation of the light emitting element 12. Since the light emitting element 12 is placed so as to face the monitor photodetector 13 and the output photodetector 15, both of the optical paths between the light emitting element 12 and the photodetector 13, and between the light emitting element 12 and the photodetector 15 are linear.
Furthermore, in the lead frame according to the present invention having the above-described structure, as shown in FIGS. 13A, 13B, 14A, 14B, and 15, the primary and secondary side lead frames 231 and 232 are combined into a laminate, and have the same reference plane when they are combined. The lead frames 231 and 232 are provided with element mounting portions 212-217 and 218-223, respectively, and turned-up or turned-down with respect to the reference plane. One of the element mounting portions extending upward does not extend over any other element mounting portion extending upward, and one of the element mounting portions extending downward does not extend below any other element mounting portion extending downward. On the element mounting portions of each of the primary and secondary side lead frames, a light emitting element 202, a monitor output element 203 and an output element 204 are placed in alternate order. When the primary and secondary side lead frames are combined, an element pair including the light emitting element and the monitor output element on the primary side faces the output element on the secondary side, and an element pair including the light emitting element and the monitor output element on the secondary side faces the output element on the primary side.
Hereinafter, functions of the present invention will be described.
According to a photocoupler device of the present invention, a light emitting element and an output photodetector are placed so as to be face each other. Thus, an optical path between the light emitting element and the output photodetector becomes linear, whereby light emitted by the light emitting element is directly incident on the output photodetector. As a result, effects of the optical path on the optical signals are small. Furthermore, the optical path is linear and short, and expensive silicone resin is not required for the formation of the optical path. Even if the silicone resin is used, a paltry amount of silicone resin is sufficient. Additionally, since the optical signal travels without using reflection, the optical signal is not likely to be affected by the external shape of the optical path. As a result, an output ratio between the monitor photodetector and the output photodetector becomes stable. Furthermore, a transmission efficiency of the optical signal between the light emitting element and the photodetector does not vary even when the state of the reflection surface is varied due to the temperature variation because the optical signal travels without using reflection. Furthermore, for the same reason, a light-shielding resin, e.g., a resin of black color, can be applied to the outermost surface of the photocoupler device, whereby interfering stray light from outside is surely blocked. As a result, reliability of the photocoupler device can be increased.
Furthermore, according to a method for fabricating a photocoupler device of the present invention, only a light emitting element is mounted on the primary side lead frame, while the monitor photodetector and the output photodetector are mounted on the secondary side lead frame. The lead frames are combined so as to provide a light emitting element and a monitor photodetector on the primary side and the output photodetector on the secondary side. In such a structure, in each lead frame, the light emitting element and respective photodetectors can be separately assembled. If assemblies of the elements are concurrently conducted, the fabrication process is simplified. After the lead frames are combined, the light emitting element and the monitor photodetector belong to the primary side, while the output photodetector belongs to the secondary side. This arrangement causes no problem in practical use.
Thus, the invention described herein makes possible the advantages of (1) providing a photocoupler device with a stable output ratio between photodetectors and a high reliability, and a fabrication method of the photocoupler device, and (2) providing a lead frame with a simple structure, superior characteristics, and improved productivity, and a photocoupler device using the same.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.